Stereophonic television sound transmission system

ABSTRACT

A compatable stereophonic television sound transmission system for transmitting left and right audio signals in conjunction with a television broadcast wherein video information is conveyed on an amplitude-modulated carrier in a frequency channel having defined frequency limits. The system includes at a transmitter location a multiplex generator for generating a composite signal having a first component representative of the sum of the audio signals, a second component comprising an amplitude-modulated suppressed carrier subcarrier signal representative of the difference between the audio signals, and a pilot component representative of the phase and frequency of the suppressed carrier. The composite signal is utilized to frequency-modulate a sound carrier to develop an RF signal component which is added to the television channel at a discrete frequency spacing from the video carrier. The system includes at a receiver location a tuner for converting the transmission channel to an intermediate frequency, a filter for separating the sound signal therefrom, and a detector for deriving the composite signal from the sound signal. The composite signal is demodulated in a stereo demodulating stage to develop the left and right audio signals. Improved noise performance is obtained in the system by applying preemphasis to the left and right audio signals, preemphasis of the composite signal, enhancement of the second composite signal component, and/or Dolby-B processing of the left and right audio signals or to the composite signal. An adapter is shown for utilizing the system for bilingual programming.

BACKGROUND OF THE INVENTION

The present application relates generally to sound transmission systems,and more particularly to a stereophonic sound transmission systemcompatible with present-day U.S. transmission standards, and toapparatus for use therein.

The transmission of stereophonic sound together with a conventionaltelevision picture transmission greatly enhances the realism andentertainment value of the program being transmitted. Various systemsand apparatus have been proposed for such transmissions includingvarious compatible subcarrier-type systems wherein left-plus-right (L+R)information is conveyed on the regular frequency-modulated sound channelof a composite television broadcast signal, and left-minus-right (L-R)information is conveyed on a subcarrier.

One such system, which was described in "Simultaneous Transmission ofTwo Television Sound Channels, " NHK Laboratories Notes, Ser. No. 132,February 1970, by Yasutaka Numaguchi, Yashitaka Ikeda, and OsamuAkiyama, conveyed L-R information on a single-sidebandamplitude-modulated subcarrier frequency-modulated on the standard NTSCaural carrier. To simplify the synchronous detection required fordemodulating the subcarrier in this system, the subcarrier was generatedat a frequency of 23.625 KHz, or one and one-half times the 15.75 KHzhorizontal scanning frequency of U.S. monochrome television broadcasts,enabling the missing subcarrier to be generated in the receiver L-Rdemodulator by sampling the horizontal deflection signal. This systemwas found to be unsatisfactory, primarily because of the insufficientsubchannel bandwidth, poor channel separation and ambiguity indevelopment of the left (L) and right (R) audio signals at the receiver.

Another system proposed for sterophonic television sound transmissionutilized a frequency-modulated subcarrier centered at 31.5 KHz, or twicethe horizontal scanning frequency. This subcarrier, whenfrequency-modulated on the NTSC-standard aural carrier, provided an L-Rbandwidth of 12 KHz. However, when it was attempted to add stereophonicdemodulation capability to the 4.5 MHz sound channel of standardintercarrier-type television receivers to recover the L-R component,video signal component contamination resulted to an extent thatsatisfactory L-R audio signals could not be obtained without extensivemodification of the receivers. Applying such subcarrier signals toconventional split-sound receivers, wherein separate intermediatefrequency (IF) channels are provided for video and sound components, isnot practical since the 41.25 MHz sound IF output of conventional modernTV tuners is above the range at which presently employed sound channelIF filters can achieve the required effectiveness.

Another system, which was proposed in U.S. Pat. No. 3,099,707 to R. B.Dome, utilized an amplitude-modulated suppressed-carrier subcarriercomponent, centered at 23.625 KHz to avoid interference with harmonicsof the horizontal scanning signal, frequency-modulated on the soundcarrier. To facilitate regenerating the subcarrier for demodulationpurposes at the receiver a 39.375 KHz pilot signal was transmittedwhich, when combined with the 15.75 KHz horizontal scanning signalpresent in the receiver, resulted in generation of the suppressed 23.625KHz carrier. This system did not provide satisfactory performance inthat the bandwidth of the L-R channel was limited to 8 KHz withsymmetrical sidebands. Attempting to increase available bandwidth by theuse of assymmetrical sidebands was not practical because this introduceda principal harmonic of the horizontal scanning signal into the uppersideband of the L-R component.

Two additional systems, which differed from those proposed in theafore-described systems in that they employed a subcarrier centered at31.5 KHz, or twice the horizontal scanning frequency, were shown in U.S.Pat. Nos. 3,046,329 to T. W. Reesor and 3,219,759 to R. B. Dome. Thefirst system was a single-sideband system which necessitated theprovision of complex filtering and demodulation circuitry in thereceiver if unacceptably narrow L-R channel bandwidth was to be avoided.The second system, like other intercarrier systems, was susceptible tovideo signal component contamination in the sound channel. Furthermore,both of these systems required connection to or at least non-destructivesampling of the horizontal deflection signal within the receiver,necessitating in the case of an add-on adapter a modification of thereceiver and the provision of an additional cable to a converter,thereby increasing installation cost and reducing the versatility of theconverter.

In contrast, the system of the present invention utilizes anamplitude-modulated double-sideband suppressed-carrier 38 KHz subcarrierL-R component frequency-modulated on the main aural carrier togetherwith a 15 KHz bandwidth L+R component and a 19 KHz pilot carrier. Thisforms a composite signal which is similar to that employed instereophonic FM broadcasts in the United States. The use of this systemsimplifies the demodulation process at the receiver, and provides asignal which is compatible with conventional nonstereophonic soundtelevision receivers. Also, the proposed system lends itself to use withself-contained converters of a design and construction which may bereadily utilized in conjunction with existing monochrome or colortelevision receivers.

Accordingly, it is a general object of the present invention to providea new and improved system and apparatus for transmitting stereophonicsound information in conjunction with a standard televisiontransmission.

It is another object of the present invention to provide a system andapparatus for transmitting stereophonic television sound which providesimproved performance and which is less subject to interference from anaccompanying video transmission.

It is another object of the present invention to provide receivingapparatus for receiving a subcarrier-type compatible stereophonictransmission which apparatus can be conveniently installed on anexisting television receiver with minimal modifications to the receiver.

It is another object of the present invention to provide a converter forreceiving a compatible stereophonic television sound transmission of thesubcarrier-type which can be used in conjunction with a conventionalstereophonic FM receiver.

It is another object of the present invention to provide apparatus forreceiving a subcarrier-type stereophonic television sound transmissionwhich can be economically constructed using standard commerciallyavailable components.

It is another object of the present invention to provide a system andapparatus for transmitting bilingual sound in conjunction with astandard television transmission.

SUMMARY OF THE INVENTION

The invention is directed to a stereophonic sound transmission systemfor transmitting left and right audio source signals in conjunction witha video source signal wherein the video source signal is modulated on anRF carrier within a television broadcast channel having definedfrequency limits. The system includes generator means including a stereomultiplex generator for generating a composite signal including a firstcomponent representative of the sum of the left and right sourcesignals, a second amplitude-modulated subcarrier componentrepresentative of the difference between the left and right signals, thesubcarrier component having upper and lower sidebands centered about asuppressed carrier, and a pilot component representative of the phaseand frequency of the suppressed carrier. Transmitter means responsive tothe composite signal are included in the system for generating afrequency-modulated signal in the broadcast channel, the centerfrequency of the signal having a predetermined spacing from thevideo-modulated signal, and receiver means for receiving the televisionbroadcast channel including means for deriving the composite signal, anddemodulator means for developing the left and right audio source signalsfrom the composite signal.

The invention is further directed to a method of transmittingstereophonic sound consisting of left and right audio source signals inconjunction with a video source signal wherein the video source signalis modulated on an RF carrier within a television broadcast channelhaving defined frequency limits. The method comprises the steps ofgenerating a composite signal including a first component representativeof the sum of the left and right source signals, a secondamplitude-modulated subcarrier component representative of thedifference between the left and right signals, the subcarrier componenthaving upper and lower sidebands centered about a suppressed carrier,and a pilot component representative of the phase and frequency of thesuppressed carrier; utilizing the composite signal to generate afrequency-modulated signal in the broadcast channel, the centerfrequency of the signal having a predetermined spacing from thevideo-modulated signal; conveying said video- and audio-modulatedsignals in the broadcast channel to a receiving location, and at thereceiving location, developing the composite signal form theaudio-modulated signal in the channel; and developing the left and rightaudio source signals from the composite signal.

The invention is further directed to a receiver for receivingstereophonic sound transmissions included on a television broadcastchannel of defined frequency limits, wherein the sound transmissioncomprise a sound carrier frequency-modulated by a composite signalincluding a first component representative of the sum of the left andright source signals, a second amplitude-modulated subcarrier componentrepresentative of the difference between the left and right signals, thesubcarrier component having upper and lower sidebands centered about asuppressed carrier, and a pilot component representative of the phaseand frequency of the suppressed carrier. The receiver includes tunermeans for converting the television broadcast channel to an intermediatefrequency channel including an intermediate frequency sound signal soundbandpass filter means for separating the sound signal from theintermediate frequency channel, sound detector means for deriving fromthe intermediate frequency sound signal a composite signal including thefirst, second and third components, and stereo demodulator means forderiving the left and right source signals from the composite signal.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the present invention which are believed to be novel areset forth with particularity in the appended claims. The invention,together with the further objects and advantages thereof, may best beunderstood by reference to the following description taken inconjunction with the accompanying drawings, in the several figures ofwhich like reference numerals identify like elements, and in which:

FIG. 1 is a functional block diagram of the transmitting portion of astereophonic television sound transmission system constructed inaccordance with the invention.

FIG. 2 is a graphic presentation of the frequency spectrum of a standardU.S. television channel.

FIG. 3 is a graphic presentation of the composite signal generated bythe stereophonic television sound transmission system of the presentinvention.

FIG. 4 is a functional block diagram of a stereophonic multiplexgenerator for use in the stereophonic television sound transmissionsystem of the invention.

FIG. 5 is a graphic presentation of the frequency spectrum of atelevision sound channel showing the effect thereon of stereophonicsound transmission in accordance with the present invention.

FIG. 6 is a functional block diagram of a single-conversion converterfor allowing reception of stereophonic television sound transmissions inaccordance with the present invention.

FIG. 7 is a functional block diagram of a stereo demodulator for use inthe stereophonic television sound transmission system of the presentinvention.

FIG. 8 is a functional block diagram of a double-conversion converterfor use in receiving stereophonic television sound transmissions inaccordance with the present invention.

FIG. 9 is a rear elevational view of a television receiver and adapterfor allowing stereophonic television sound transmissions in accordancewith the present invention to be received by the receiver.

FIG. 10 is a functional block diagram of the stereophonic televisionsound adapter shown in FIG. 9.

FIG. 11 is a rear elevational view of a television receiver and adapterfor adapting the receiver to receive stereophonic television soundtransmissions in accordance with the invention.

FIG. 12 is a functional block diagram of the stereophonic televisionsound adapter shown in FIG. 11.

FIG. 13 is a functional block diagram of a television receiverincorporating means for receiving stereophonic television soundtransmissions in accordance with the present invention.

FIG. 14 is a functional block diagram of a converter for allowingreception of stereophonic television sound transmissions in accordancewith the present invention on a standard stereophonic FM broadcastreceiver.

FIG. 15A is a schematic diagram partially in functional block form ofthe transmitter portion of a system for bilingual television soundtransmission in accordance with the invention.

FIG. 15B is a schematic diagram partially in functional block form ofthe receiver portion of a system for bilingual television soundtransmission in accordance with the invention.

FIG. 16 is a graphic presentation of various transmission standards asapplicable to a stereophonic television sound transmission systemconstructed in accordance with the invention.

FIG. 17A is a schematic diagram partially in functional block form ofthe transmitter portion of a system for L-R component enhancement inaccordance with the invention.

FIG. 17B is a schematic diagram partially in functional block form ofthe receiver portion of a system for L-R component enhancement inaccordance with the invention.

FIG. 18A is a functional block diagram of the transmitter portion of thestereophonic television sound transmission system of the inventionincorporating means for L-R component enhancement for improvedperformance.

FIG. 18B is a functional block diagram of the receiver portion of thestereophonic television sound transmission system of the inventionincorporating means for compensating for L-R component enhancement.

FIG. 19A is a functional block diagram of the transmitter portion of thestereophonic television sound transmission system of the inventionshowing means for Dolby type B encoding incorporated therein.

FIG. 19B is a functional block diagram of the receiver portion of thestereophonic television sound transmission system of the inventionshowing means for Dolby type B decoding incorporated therein.

FIG. 19C is a functional block diagram of a Dolby type B signalprocessing stage suitable for use in the stereophonic television soundtransmission system of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the Figures, and particularly to FIGS. 1-6, a stereophonicsound transmission system constructed in accordance with the inventionmay be employed in conjunction with an aural transmitter 20 and a visualtransmitter 21, which may be conventional in design and construction.The radio frequency (RF) output signals from the two transmitters arecombined in a conventional RF signal diplexer 22 and radiated by meansof a common transmitting antenna 23.

Video source signals such as may be obtained from cameras, film chains,video tape recorders or the like, are applied to the visual transmitter21 to produce an amplitude-modulated (AM) RF output signal 24 having thebandpass characteristic shown in FIG. 2. At the same time, left (L) andright (R) stereophonic audio signals, such as may be obtained frommicrophones, tape decks, turntables, or the like, and which typicallyrepresent the sound level at two different locations in the scene beingtelevised, are applied to the aural transmitter 20. Within thistransmitter these signals are applied to an stereo multiplex generator25, which may be similar in construction and operation to those utilizedin conventional stereo FM broadcast transmitters. The output ofgenerator, in accordance with the invention, consists of a compositesignal which includes a direct L+R audio component 27, a double-sidebandL-R component 28 consisting of upper and lower sidebands 28a and 28bcentered at 38KHz, and a 19 KHz pilot component 29, as shown in FIG. 3.This signal is applied to conventional RF modulater and amplifiercircuits 30 within aural transmitter 20 to develop a frequency modulatedRF output signal 26 centered 4.5 MHz from the video signal, as shown inFIG. 2.

Referring to FIG. 4, the multiplex generator 25 may include a pair ofpre-emphasis networks 31a and 31b and a pair of 17 KHz low pass filters33a and 33b for the L and R channels, respectively. As is well known tothe art, the pre-emphasis circuits impose a frequency responsecharacteristic on the L and R signals which emphasizes the higherfrequencies to improve the signal-to-noise ratio of the transmittedprogram. The low pass filters 33a and 33b serve to prevent input signalsexceeding 17 KHz from affecting the 19 KHz pilot and L-R components. Theleft and right audio signals from filters 33a and 33b are applied to asynchronous switching stage 35 wherein they are alternately sampled todevelop the L+R and L-R components in a manner well known to the art.The operation of switching stage 35 is controlled by a 38 KHz squarewave signal, which is generated by means of a 76 KHz crystal-controlledoscillator stage 36 and a 2:1 frequency divider stage 37.

After further filtering in a 60 KHz low pass filter 38 to removeharmonics which may exist in the composite signal above 53 KHz, theoutput of the synchronous switching stage 35 is combined with a 19 KHzpilot signal in a summing stage 39. The pilot signal is derived by meansof an additional 2:1 frequency divider stage 40 and 19 KHz low passfilter and phase adjustment stage 41 to assure precise time coincidencewith the 38 KHz sampling action of switching stage 35.

Referring to FIG. 3, in the basic system contemplated by the inventionthe L+R signal component 27 generated by stereo multiplex generator 25preferably has a frequency range extending from 50 Hz to 15 KHz and anamplitude sufficient to produce a maximum sound carrier frequencydeviation of 22.5 KHz. The L-R signal component 28 consists of lower andupper side band components 28a and 28b preferably centered about a 38KHz suppressed carrier and extending from 23 to 37.95 KHz and 38.05 to53 KHz, respectively, each having an amplitude sufficient to produce amaximum frequency deviation of 11.25 KHz in the sound carrier. The 19KHz pilot component 29, which is preferably centered between the lowersideband 28b and the L+R component 27, is transmitted at a frequencydeviation of 2.5 KHz in the sound carrier. As shown in FIG. 3, for theillustrated embodiment the total bandwidth required by the compositesignal is 53 KHz and, by reason of the amplitude limitations imposed onthe L+R, L-R, and pilot components, the maximum deviation of the soundcarrier is 25 KHz.

In accordance with one aspect of the invention, the frequency of thepilot carrier may be increased to 5/4 the horizontal scanning rate ofthe video transmission (Fh), or 19.6875 KHz in the case of U.S.monochrome transmission and 19.66783 KHz in the case of U.S. colortransmissions. This centers the suppressed carrier between the secondand third harmonics of the horizontal scanning frequency, whichharmonics have been found to be a principal cause of interference inprior-art stereophonic sound systems which lacked adequate videocomponent rejection. This also reduces interference to the L-R componentto a single component at approximately 7.8 KHz instead of threecomponents at 6.5 KHz, 9.25 KHz and 2.75 KHz (beat between the 6.5 KHzand 9.25 KHz components perceived by a listener) as with a 19 KHz pilotcarrier.

A stereophonic sound converter 50 for receiving stereophonic soundtransmissions in accordance with the invention is shown in FIG. 6. Thisconverter operates independently of the television receiver, having aninput for direct connection to a conventional television antenna 51 andL and R audio outputs for connection to an external stereo amplifier andspeaker system. The RF signals intercepted by antenna 51 are applied toa user-adjustable tuner 52 within the converter wherein the desiredtelevision channel is selected, amplified and converted to a suitableintermediate frequency, in this case 10.7 MHz. The intermediatefrequency (IF) signal is applied to an IF amplifier stage 53, whereinadditional amplification and limiting are provided. The amplified IFsignal is applied to a conventional FM detector stage 54 wherein acomposite audio signal having L+R, L-R and pilot signal components asdepicted in FIG. 3 is developed in a manner well known to the art. Inaddition, detector 54 may also develop an automatic frequency control(AFC) voltage for application to tuner 52 to maintain the tuner properlytuned to the desired station, and the IF amplifier stage 53 may developan AGC signal which is applied to tuner 52 to maintain a constant signallevel.

In order to obtain the L and R audio signals necessary for driving astereo amplifier and speaker system the composite signal from detector54 is applied to a stereo demodulator 55, which serves as thecounterpart of the stereo multiplex generator 25 in the transmitter. Inits most basic form the demodulator 55 may include a commercialintegrated-circuit (IC) type stereo demodulator circuit similar to thosecommonly employed in stereo FM broadcast receivers together withnecessary de-emphasis circuits for the L and R audio outputs of thecircuit. Referring to FIG. 7, within such a demodulator circuit thecomposite signal is typically amplified by a buffer amplifier 60 andapplied to a phase-detector 61, which comprises part of a phase-lockloop. The phase-lock loop includes a low-pass filter 62, a DC amplifier63, a voltage-controlled 76 KHz oscillator 64, two 2:1 frequency dividerstages 65 and 66, and a phase correction circuit 67 whose output ispresented as a second input to the phase detector 61 for comparison withthe composite input signal. The phase-lock loop is designed to lock ontothe 19 KHz pilot carrier and product its 38 KHz second harmonic incorrect phase to control synchronous switch stage 68.

The synchronous switch 68 alternatively samples the composite stereoinput signal at a 38 KHz rate, synchronized precisely in time and in thesame sequence as the corresponding samples are assembled by thesynchronous switch 35 in stereo multiplex generator 25 (FIG. 1) informing the composite signal at the transmitter. As is well known to theart, this results in the L and R audio signals being developed at theoutput of the switch, and these derived audio signals are applied torespective ones of two de-emphasis networks 69a and 69b wherein apredetermined frequency response characteristic is introduced tocompensate for the pre-emphasis characteristic introduced at thetransmitter. The L and R audio outputs of the synchronous switch may beapplied to an external stereo amplifier and speaker system, oralternatively applied to suitable audio amplifier stages and/or speakersprovided within the converter.

A dual-conversion version of the stereophonic television sound converteris shown in FIG. 8. In this embodiment turner 52 converts the selectedtelevision broadcast signal to an IF signal which includes a videocomponent centered at 45.75 MHz and a sound component centered at 41.25MHz. This signal is applied to a 41.25 MHz bandpass filter 70 whereinthe sound component is separated and applied to a mixer stage 71. Withinmixer state 71 the IF sound component is combined with a 51.95 MHz, oralternatively, a 30.55 MHz continuous wave signal from an oscillatorstage 72 to develop a second IF signal at 10.7 MHz. This signal isamplified and amplitude-limited in a conventional 10.7 MHz IF amplifierstage 53 prior to application to an FM detector 54, wherein it isconverted to a composite stereo signal having L+R, L-R and pilot signalcomponents as depicted in FIG. 3. In addition, as in the previouslydescribed single-conversion converter of FIG. 6, IF amplifier stage 53may develop an AGC voltage for application to turner 52 and detector 54may develop an AFC voltage for centering the IF frequency, the AFCvoltage being applied to either oscillator 72 or tuner 52.

Image rejection for the dual-conversion converter is superior to thatfor the 10.7 MHz IF single-conversion converter, in that the 41.25 MHzfirst intermediate frequency provides greater separation between thefrequency of the received signal and the image frequencies to which thereceiver is subject. With the 41.25 MHz IF channel the receiver issubject to a primary image band 82.5 MHz removed from the receivedbroadcast and a potential secondary interference band 20.625 MHz removedfrom the broadcast which can be doubled in the mixer and thereby passthrough the IF amplifier. Fortunately, frequencies that far removed areefficiently rejected by normal tuner selectivity in the dual-conversionconverter. However, in the single-conversion converter these primary andsecondary frequency bands are removed from the described signal by only21.4 MHz and 5.35 MHz respectively, the proximity of the latter beingsuch that a portion of the video spectrum of the selected channel and anadjacent channel may pass through the IF amplifier to contaminate thesound channel. Therefore, the tuner for the single-conversion convertermust exhibit far greater selectivity than that utilized in thedual-conversion converter. The problem of discriminating againstsecondary interference in the single-conversion receiver may beameliorated by adopting an IF slightly greater than 10.7 MHz.

For convenience, it may be desirable to perform channel selection at theconverter for both the sound and video portions of a broadcast. To thisend, the converter may take the form of an adapter 90 such as than shownin FIGS. 9 and 10. This adapter includes suitable RF circuitry forreceiving both the audio and video portions of the signal, and forconcurrently supplying the converted video IF signal to a conventionaltelevision receiver 94 to permit reproduction of the video scene. Theadapter 90, which, except for an additional RF output circuit extendingto the television receiver, may be similar in design and construction tothe converter shown in FIG. 8, includes a tuner 92 for convertingsignals intercepted by the television receiving antenna 51 to anintermediate frequency. The intermediate frequency signals,corresponding to those commonly employed in a television receiver, i.e.41.25 MHz for the sound carrier and 45.75 MHz for the video carrier, areamplified in an RF amplifier 92 and coupled through a coaxial cable 93to the television receiver 94. Within the television receiver thecoaxial cable 93 may terminate in an isolation network 95 which servesto couple the signals to UHF input of the VHF tuner 96a of thetelevision receiver. The VHF tuner is coupled in a conventional mannerto the main chassis 99 of the receiver, which may be conventional in allrespects. The main chassis develops a video output signal for driving apicture tube 97.

The balance of the stereophonic sound converter 90 is similar inconstruction and operation to the converter shown in FIG. 8. As in theconverter of FIG. 8, the 41.25 MHz output of RF amplifier 92 is appliedthrough a 41.25 MHz bandpass filter 70 to the input of mixer stage 71.There, the IF signal is heterodyned with a continuous wave 51.95 MHzsignal developed by an oscillator 72. As a result a 10.7 MHz IF signalis developed which is applied to the 10.7 MHz IF amplifier 53. Theamplified IF output signal from this stage is applied to an FM detector54 wherein a composite audio signal having L+R, L-R and pilot componentsis derived. The composite audio signal is applied to a stereodemodulator stage 55 wherein L and R audio signals are developed forapplication to external stereophonic amplifying equipment (not shown).

In operation, tuner 91, which may consist of a conventional turrent orbandswitch type discrete-channel tuner of the type commonly incorporatedin consumer television receivers, is set to a desired channel and theintermediate frequency output from the tuner is routed through RFamplifier 92, cable 93 and isolation network 95 to the input of thetelevision receiver VHF tuner 96a. This interconnection can usually bereadily accomplished, since the VHF tuner 96a is ordinarily connected tothe UHF tuner 96b by means of a coaxial cable 93a having plugs on atleast one end, so that it is only necessary to unplug this cable andplug in the isolation network to complete the connection. The isolationnetwork serves to isolate or decouple chassis grounds as well as tomatch impedances. VHF tuner 96a, when switched to its UHF position,serves only to pass the signal from the adapter 90 to the televisionmain chassis 99.

Adapter 90 develops L and R audio signals while the television receiver94 operates in a normal manner to product a picture on a picture tube97. Since the operation of the television receiver has no effect uponthe reception of the stereophonic sound signal, instability or poorsignal quality within the receiver cannot depreciate the quality of thereproduced sound. The automatic gain control (AGC) circuits of thetelevision receiver remain in effective control of video level with thisarrangement, and while tuner 91 is adjusted to optimize picture qualityin television receiver 94, the quality of the reproduced sound isautomatically and independently optimized by AFC and AGC circuits of theadapter.

It will be appreciated that instead of the multichannel tuner, it isalso possible to utilize a single-channel tuner for receiving a specialinterest channel. Obviously, this arrangement results in simplificationand reduced manufacturing costs for the adapter, making the packageattractive for promotional and special interest uses.

A version of the stereophonic sound adapter for use in conjunction witha conventional television receiver, wherein channel selection isaccomplished within the receiver, is shown in FIGS. 11 and 12. Thisarrangement allows the option of concealing the adapter 100 within thetelevision receiver cabinet, as shown in FIG. 11. The televisionreceiver, as shown in FIG. 12, may be conventional in design andconstruction, comprising a receiving antenna 51, a tuner 101, televisionreceiver circuits 103, a picture tube 104, and a loudspeaker 105. Tofacilitate operation of the adapter the intermediate frequency outputsignal from tuner 101 is non-destructively sampled by a pick-up 102 andconveyed through a coaxial cable 106 to the input of a variable-gain RFamplifier 107. The pick-up 102 may consist of a high-impedance voltagepick-up coupled to the signal path, or alternatively a low-impedancecurrent pick-up which may be inserted in series with the signal path byunplugging the existing cable between the tuner and the main chassis andplugging in the pick-up.

The output of RF amplifier 107 is applied through a 41.25 MHz soundbandpass filter 108 to a first mixing or heterodyning stage 109, whereinthis signal is heterodyned with a 51.95 MHz continuous wave signalsupplied by an oscillator 112 to develop a 10.7 MHz IF signal. As in thepreviously described converters, this signal is applied to a 10.7 MHz IFamplifier 113 wherein it is amplified and amplitude-limited, and fromthere to a conventional FM detector stage 114. The composite outputsignal from detector 114 is applied to the non-inverting input of adifferential amplifier 118, whose output is in turn applied to a stereodemodulator stage 119 to develop L and R audio output signals forconnection to an external stereophonic audio amplifier system.

To provide improved performance, an optional 45.75 MHz video bandpassfilter 110 may be connected to the output of amplifier 107. The videosignal passed by this filter is mixed in a second mixing or heterodyningstage 111 with the 51.95 MHz continuous wave signal developed byoscillator 112 to form a 6.2 MHz IF signal. The 6.2 MHz signal isamplified and amplitude-limited in a 6.2 MHz IF amplifier 115 andapplied to a conventional FM detector stage 116 wherein an output signalindicative of frequency shift in the video channel is developed. Theoutput signal is applied through a 500 Hz low-pass filter 117 to theinverting input of differential amplifier 118, causing the output ofthis amplifier to correspond to the difference between the compositesignal from detector 114 and the low frequency signal from detector 116.

The effect of subtracting the low frequency audio component optionallyderived from the 6.2 MHz IF signal is to cancel out some or all of theeffects of any extraneous FM modulation which exists in the soundchannel at the output of television receiver tuner 101 as a result ofmicrophonics or AC power supply harmonics. The 6.2 MHz circuits areeffective for this purpose because the video carrier is relatively freeof frequency modulation components below 500 Hz, therefore any suchfrequency modulation finding its way into FM detector 116 is necessarilydue to an undesirable effect common to both signal paths, and thereforeshould be cancelled out of the principal sound channel by differentialamplifier 118.

The necessary circuitry for receiving stereophonic television soundbroadcasts transmitted in accordance with the invention may also beprovided as an integral part of a television receiver, as shown in FIG.13. In the illustrated receiver, which except for its sound channel maybe conventional in structure and operation, television transmissions areintercepted by an antenna 51, and amplified and converted by aconventional turner 120 to an intermediate frequency. The 45.75 MHzvideo portion of the IF signal is amplified by a 45.75 MHz video IFamplifier 121, and then applied to a video detector 122 wherein videoinformation in the intercepted signal is derived. The video signal fromdetector 122 is amplified in a conventional video amplifier stage 123and applied to a picture tube 124 to control the brightness of theelectron beam thereon. The horizontal and vertical scanning of theelectron beam is controlled by conventional deflection circuits 125which receive synchronizing pulses from video detector 122.

The sound signal appears at the output of tuner 120 as a 41.25 MHz IFsignal. This signal is separated from the 45.75 MHz video signal by a41.25 MHz sound bandpass filter 126 and applied to a mixing orheterodyning stage 127. In mixing stage 127 the 41.25 MHz sound Ifsignal is combined with a 30.55 MHz, or alternatively a 51.95 MHz,continuous wave signal from an oscillator 128 to develop a 10.7 MHz IFsignal. This signal is amplified and amplitude limited in a 10.7 MHz IFamplifier stage 129 and applied to an FM detector 130 wherein acomposite signal containing L+R, L-R and pilot components as depicted inFIG. 3 is developed. The composite signal is applied to a stereodemodulator 131 wherein L and R audio signals are developed. FM detector130 also develops an AFC voltage which is applied to appropriatefrequency control circuitry in oscillator stage 128 to maintain the 10.7MHz IF signal centered in the IF channel regardless of the fine tuningof tuner 120.

The stereo demodulator 131, which contains both stereo demodulation andde-emphasis circuitry, such as those described in connection with thepreviously described stereophonic sound converters and adapters,reproduces from the composite signal the R and L audio signals developedat the program source. These audio signals are applied to respectiveinputs of audio amplifiers 132 and 133 wherein they are amplified to alevel suitable for driving respective loudspeakers 134 and 135.Preferably, these spakers are located to the right and left of picturetube 124 as shown to provide a realistic stereo effect during viewing ofthe television receiver.

The sound channel of the television receiver is dual-conversion indesign, the first conversion stage being contained in the tuner 120. Forthis application, with present technology, a single-conversion soundchannel would be substantially inferior by reason of the high IF outputfrequency (41.25 MHz) of available television tuners, and the difficultyof building filters, amplitude-limiters, and FM detectors capable ofoperating at that frequency while meeting the stringent requirements ofthe IF amplifiers for high-fidelity stereophonic sound. Combinedbandwidth, pass-band phaselinearity, and skirt attenuation designrequirements are outside of practical technical and/or consumer marketeconomic ranges, using present-day RLC, ceramic, and crystal filters,although it is contemplated that new filter technology may ultimatelymeet these filter requirements. Lowering the 41.25 MHz output frequencyof modern television tuners is not an attractive alternative, sincesuperior image rejection and adequate video channel bandwidth areimportant advantages of the higher IF frequency.

A dual-conversion sound channel retains the superior image-rejectionadvantage of the standard 41.25 MHz tuner output frequency, whilesimultaneously exploiting the advantages of a low second-conversion IFoutput frequency to achieve improved limiting and FM detection.Furthermore, a dual-conversion sound channel more effectively isolatesthe video and sound channels while independently optimizing both bymeans of AFC and AGC signals derived in the respective channels. It willbe appreciated that frequencies other than 10.7 MHz may be utilized forthe second IF channel for optimum performance, the principal advantageof the 10.7 MHz frequency being for the present the ready availabilityof 10.7 MHz IF amplifier components.

In accordance with another aspect of the invention, stereophonictelevision sound signals transmitted in accordance with the inventioncan be received by a conventional FM stereo broadcast receiver by meansof the adapter 80 shown in FIG. 14. The converter includes an RFammplifier 81 to which the RF signal intercepted by the receivingantenna 51 is applied, and a mixer stage 82, wherein the amplifiedsignal is heterodyned with a continuous wave output signal from anoscillator 83. The RF amplifier 81, mixer 82 and oscillator 83 togetherfunction as a tuner 84, the operating frequency of RF amplifier 81 beingadjusted to the desired television broadcast channel and the frequencyof oscillator 83 being adjusted to operate at a frequency removed fromthe television channel sound carrier such that the sound differencefrequency, when tripled, falls within the 88-108 MHz FM broadcast band.In the illustrated embodiment this intermediate frequency is 30 MHz.

The intermediate frequency output signal from mixer 82 is applied to a30 MHz IF amplifier stage 85 wherein it is amplified andamplitude-limited prior to being applied to a tripler and 90 MHz filterstage 86. To maintain the converter 80 centered on the desired channelthe 30 MHz output signal from If amplifier stage 85 may be applied to anFM detector 87 to develop an AFC signal for application to oscillator83. The output of tripler 86, which constitutes a stereophonic signalhaving modulation characteristics similar to those of a standardstereophonic FM broadcast signal, is applied to the antenna inputterminal of a conventional FM stereo tuner (not shown). The output ofthe FM stereo tuner, which consists of L and R audio output signals, maybe applied to a conventional stereo amplifier, and then to left andright loudspeakers which preferably are placed on either side of thetelevision screen on which the video portion of the received broadcastis being viewed. A selector switch (not shown) may be included in theoutput circuitry of adapter 80 to facilitate connecting the FM tuner toan FM receiving antenna (not shown) when the adapter is not in use.

Since the frequency deviation of the third harmonic of the 30 MHz IFsignal is three times the 25 KHz maximum deviation of the TV soundcarrier, the 75 KHz maximum deviation prescribed for standard FMbroadcasts is obtained in the resulting 90 MHz signal. For example,assuming reception of TV channel 11, the sound carrier of the receivedsignal is located at 203.75 MHz and the video carrier is at 199.25 MHz.This dictates an oscillator frequency of 233.75 MHz, resulting in anintermediate frequency video carrier at 34.5 MHz and a sound carrier at30 MHz. The 34.5 MHz video carrier is eliminated in the IF amplifierstage, leaving only the 30 MHz sound carrier for tripling to 90 MHz intripler 86, and reception on FM broadcast channel 211 at 90 MHz. Sincethe pilot is, in accordance with the invention, established at 19 KHzthe same demodulator circuits utilized in the tuner for demodulatingstandard FM broadcasts serve to demodulate the stereophonic televisionsound signal.

Filters for use in the 30 MHz IF amplifier 85 are within the practicaldesign capabilities of recent surfacewave technology, and provide aparticularly good application for a filter of (sin X/X) configuration.FM detection at the 30 MHz frequency is not a problem in thisapplication, since that function is performed externally within thestereo tuner.

The technique of increasing frequency deviation by utilizing a harmonicof the desired signal provides the basis for improving the performanceof the limitor and discriminator stages of an FM receiver. This isbecause increasing the frequency deviation of the modulated intermediatecarrier effectively increases the level of the developed output signal.To illustrate application of this technique, the second conversion from41.25 MHz to 10.7 MHz in the previously described television soundconverter of FIG. 8 can be accomplished by selecting 46.60 MHz as thefrequency of oscillator 72, thereby containing a difference frequency of5.35 MHz at the output of mixer 71. The 10.7 MHz IF amplifier 53, beingnow tuned to the second harmonic of 5.35 MHz, provides twice thefrequency deviation of the transmitted signal to the FM detector 54. Theamplitude of the 10.7 MHz second harmonic thus extracted need not equalthe amplitude of the 5.35 MHz fundamental to receive the full benefit ofthe increased deviation for maximum signal-to-noise improvement. Allthat is required is that it exceeds the minimum threshold level of IFamplifier 53 so that good limiting action is obtained. It should beobvious to those skilled in the art that by designing for other suitablylower difference frequency outputs from mixer 71, still higher orderharmonics can be extracted by the 10.7 MHz IF amplifier, yieldingproportionately increased frequency deviations.

In accordance with another aspect of the invention, the stereophonictelevision sound system of the invention can be utilized for bi-lingualprogramming. As shown in FIG. 15A, assuming that the sound portion of atelevision broadcast is to be broadcast simultaneously in two differentlanguages A and B, the A sound source is connected through resistances150 and 151 to the inverting inputs of first and second differentialamplifiers 152 and 153, respectively. The B sound source is connectedthrough a resistance 154 to the inverting input of amplifier 152 andthrough a resistance 155 to the non-inverting input of amplifier 153.The non-inverting inputs of amplifiers 152 and 153 are connected toground by resistances 156 and 157, respectively, and the invertinginputs are connected to the outputs of their respective amplifiers byresistors 158 and 159, respectively. The outputs of amplifiers 152 and153 are connected to the L and R audio inputs of the system stereogenerator 147, which may be identical in construction and operation tothe stereo generator 25 shown in FIG. 4.

As a result of this matrixing arrangement language B modulates what wasformerly the 38 KHz L-R sub-carrier channel, and language A modulateswhat was formerly the L+R main channel. The 19 MHz pilot component istransmitted as it was during the transmission of stereophonic programmaterial.

At the receiver, as shown in FIG. 15B, and L and R audio outputs of thesystem stereo demodulator 148 are applied through respective resistances160 and 161 to the inverting and non-inverting inputs of a differentialamplifier 162. The output of amplifier 162 is coupled back to theinverting input terminal by a resistance 164 and the non-inverting inputis connected to ground by a resistance 163. Resistances 160, 161, 163and 164 form a matrix in combination with amplifier 162 to generate asignal corresponding to language B at the output of the amplifier.Language A can be obtained at either of the output terminals of thestereo demodulator 148 by conditioning the demodulator for monophonicoperation. A three-pole three-position mode selection switch 165 may beprovided to select the signal to be amplified by an external two channelaudio amplifier 166 and applied to loudspeakers 167 and 168, and tocondition the demodulator for monophonic operation during reception oflanguage A.

With this arrangement, it is contemplated that language A would normallybe the majority or domestic language, since the L+R channel on which itis conveyed is compatibly received by existing monaural televisionreceivers. At the receiving end the matrixing circuitry can beconstructed as an adapter 149 which can be readily added to orincorporated in existing receivers, such as those depicted in FIGS. 6and 8, to enable selective reproduction of either language A or languageB. It should also be noted that the bi-lingual system can also be usedin conjunction with standard FM stereo broadcasts. In this case theadapter 149 is connected between the L and R audio outputs of the stereoFM receiver and the stereo amplifying system.

From the preceding discussion it will be realized that the basicstereophonic television sound transmission system contemplated by thisinvention requires only the addition of a stereo FM multiplex generatorto existing television sound transmission equipment, and the addition ofa converter or adapter to existing television receiving equipment.However, by modifying certain parameters of the heretofore describedsystem in accordance with further aspects of the invention to besubsequently described, improved sound transmission is possible inconjunction with such existing equipment. Such modifications arefeasible at this time since commercial stereophonic televisionbroadcasts are presently non-existent, and engineering standardsconcerning such broadcasts have not been established. Therefore, inanticipation of, and as a basis for establishing such standards, it isappropriate to examine the characteristics of the modulated soundcarrier generated by the transmission system in detail to determine whatstandards provide for optimum transmission of stereophonic sound withoutdetriment to picture quality.

Referring to FIG. 3, the maximum frequency deviation of either L-Rcomponent is 50% that of the L+R main channel component, this reductionbeing the result of the L-R energy being spread over two sidebands whichspan twice the bandwidth of the main channel. This has the effect ofreducing the modulation indices of the L-R channel relative to the mainchannel. Moreover, the L-R channel modulation indices are furtherreduced by the well known 1/f decrease of the modulation index withincreasing modulation frequency. This is graphically illustrated inlogarithmic format by FIG. 16, wherein Curve A is a plot of modulationindex vs. modulation frequency (measured from the sound carrier) for thecondition of constant maximum frequency deviation (22.5 KHz). Curve B isa similar plot, except that, in accordance with the above-mentionedmaximum frequency deviation limits of FIG. 3, the modulation indices ofthe L-R channel are depressed 50% (6 db.), while the L+R region remainsidentical to Curve A.

Modulation index curve B represents modulation at the 100% level for theproposed transmission system based upon a uniform audio spectral energydistribution. In practice the distribution of energy peaks in audioprogram material falls off with increasing frequency. This is shown byCurve C, wherein only lower frequency peaks attain the 100% modulationlevel of curve B. The form of curve C is actually that of a de-emphasisnetwork having a time constant of 25 microseconds, that curve havingbeen found to best approximate the energy distribution in modern audioprograms as shown by Ray M. Dolby, Optimum Use of Noise Reduction in FMBroadcasting, Journal of the Audio Engineering Society, Vol. 21, No. 5,June 1973, and D. P. Robinson, Dolby B-Type Noise Reduction for FMBroadcasts, Journal of the Audio Engineering Society, Vol. 21, No. 5,June 1973. These references demonstrate that the conventional 75microsecond time constant presently prescribed by U.S. FM radiostandards is outmoded, being based on the frequency distribution ofprogram material as it existed at an earlier time using equipment andmethods which are now obsolete.

Referring again to FIG. 16, curves X and Y depict the effect onfrequency response of pre-emphasis networks having respective timeconstants of 75 and 25 microseconds in the L+R region, the L-R regionhaving been omitted for reasons of clarity. Curve D illustrates theeffect of a 75 microsecond pre-emphasis network on modern programmaterial (as represented by curve C). Curve D is obtained by subtractingcurve C from curve X, with curve B as the baseline. Over-modulation isthat portion of curve D which exceeds the 100% modulation line (curveB), being prominent at high audio modulation frequencies of both the L+Rand L-R bands.

It can be concluded from curve D that the result of applying the 75microsecond pre-emphasis required by U.S. standards in present-day FMbroadcasting has been overcompensation of the high frequencies,requiring either amplitude limiting of high frequencies, or substantialunder-modulating of mid and low frequencies to avoid over-modulation ofthe transmitter. The penalty in the first instance is diminished highfrequency response when the program material is de-emphasized prior toreproduction at the receiver. The penalty in the latter instance isreduced broadcast coverage.

In interpreting curve D it should be considered that the ordinate inFIG. 16 is the phase modulation angle of the sound carrier, which may betaken as an indication of tolerance by the transmitted signal to noiseinterference along the transmitter-receiver radiation path. Curve Dreveals that, when 75 microsecond pre-emphasis is applied, the overallnoise tolerance of the L-R sidebands is substantially inferior to thatof the main channel, the modulation frequencies in the region of 38 KHzbeing particularly deficient. This latter deficiency, together withover-modulation of high audio frequencies, are present-day problems ofFM stereo broadcasting.

If 75 microsecond pre-emphasis were to be adopted as a standard forstereophonic television sound, the problem of L-R noise susceptibilitywould be more serious than is presently the case for FM stereobroadcasts, since FM broadcast standards specify 75 KHz as the maximumfrequency deviation, whereas the corresponding maximum specified fortelevision sound transmission is only 25 KHz. For comparison, curve E,which results from subtracting curve C from curve Y, with curve B as thebaseline, shows the effect of applying 25 microsecond pre-emphasis tothe representative program content of curve C. It will be noted thatcurve E coincides at all frequencies with curve B, which can beinterpreted as indicating that, with 25 microsecond pre-emphasis, theprogram can be transmitted with essentially 100% modulation at allfrequencies. It is also apparent that adopting 25 microsecondpre-emphasis eliminates the 38 KHz-centered noise tolerance deficiencyand high frequency over-modulation characteristics of 75 microsecondpre-emphasis. However, the overall L-R noise susceptibility (relative tothat of the L+R main channel) remains low, as evidenced by the 9.6 dbdrop between the low point (15 KHz) of the L+R channel and the highpoint (23 KHz) of the L-R channel, as shown in FIG. 16.

One way to increase L-R noise tolerance is shown by curve F, wherein L-Rsignal amplitude has been increased by a factor of 3 (9.6 db) relativeto curve E, while the L+R channel amplitude remains unchanged. Anotherway to improve L-R noise susceptibility is illustrated by curve G, whichresults from first enhancing the L-R portion of curve E by a factor of 2(6 db), 7.5 microsecond preemphasis to the overall stereo compositesignal. This raises the relative modulation level of the stereo channeland adjusts the slope to increase the relative signal strength at thehigh frequency end of the L-R spectrum. Since preemphasis having a 7.5microsecond time constant has its greatest effect above approximately 20KHz, its effect on the L+R main channel is minimal.

Thus, in accordance with further aspects of the invention, the basictransmission system of the invention may be significantly improved withrespect to sound fidelity, broadcast coverage, and signal-to-noise ratioof the L-R channel, by 1) employing an audio pre-emphasis time constantof 25 microseconds rather than the 75 microsecond time constant requiredby the present FM and TV broadcast standards, 2) enhancing only the L-Rcomponent of the composite stereo signal, while maintaining the L+R mainchannel unchanged, and 3) applying additional pre-emphasis to thecomposite signal in combination with selective enhancement of the L-Rregion to adjust or eliminate, as desired, the negative slope of the L-Rspectrum.

FIG. 17A shows circuitry for enhancing the amplitude of the L-R channelwithout affecting the L+R main channel, thereby achieving the improvedsignal-to-noise performance characteristic of curve F in FIG. 16. Inthis further aspect of the invention, additional circuitry 140 isinterposed between the L and R program sound sources and the L and Raudio inputs of the stereo multiplex generator 25 at the studio.Alternatively, this additional circuitry could be incorporated withinthe multiplex generator itself, the internal pre-emphasis circuits ofwhich may be converted to a 25 microsecond time constant. At thereceiver additional circuitry 141, which compensates for the effect ofthe transmitter circuitry 140, is added as shown in FIG. 17B. Thereceiver compensating circuitry 141 may be a simple adapter connectedbetween the L and R audio output terminals of the receiver and thecorresponding inputs of an external stereo amplifier/speaker system, ormay be incorporated within the stereo demodulator 55 of the receiver.

The enhancement circuit shown in FIG. 17A utilizes two differentialamplifiers 142 and 142. Amplifier 142 has its non-inverting inputconnected to the L sound source and amplifier 143 has its non-invertinginput connected to the R sound source. The output of amplifier 142,henceforth designated L', is connected to the left audio input of themultiplex generator 25, and by an impedance Za to its inverting input.The output of amplifier 143, henceforth designated R', is similarlyconnected to the right audio input of the multiplex generator 25 and byan impedance Zb to its inverting input. The inverting inputs ofamplifiers 142 and 143 are interconnected by an impedance Zc.

If the three impedances are arranged to be resistive and of equal value(Za = Zb = Zc), the L-R audio amplitude increases by a factor of 3,while the L+R audio amplitude remains unchanged. This follows since##EQU1##

The relationship is shown in the following tabulation for various inputcombinations wherein each unit corresponds to an 11.25 KHz deviation:

                  TABLE 1                                                         ______________________________________                                        L    R      L'     R'   L + R L' + R'                                                                              L - R L' - R'                            ______________________________________                                        1    1      1      1    2     2      0     0                                  1    -1     3      -3   0     0      2     6                                  1    0      2      -1   1     1      1     3                                  0    1      -1     2    1     1      -1    -3                                 ______________________________________                                    

As shown in FIG. 17B, a compensating circuit 141 consisting of threeimpedances Za', Zb', and Zc' and a pair of audio amplifiers 144 and 145may be provided at the receiver to restore the l and R audio signals tothe amplitude relationship they had prior to the L-R enhancementintroduced at the transmitter. The L' audio output signal fromdemodulator 55 is coupled to the input of audio amplifier 144 byimpedance Za' and the R' audio output signal from the demodulator iscoupled by impedance Zb' to the input of audio amplifier 145. The inputsof amplifiers 144 and 145 are connected together by impedance Zc'. As inthe previously described embodiments, the composite signal developedwithin the converter or adapter is applied to demodulator 55.

If, as illustrated above, Za, Zb, and Zc are made equal and resistive inthe enhancement circuit, Za', Zb', and Zc' will also necessarily beequal and resistive, although they need not have the same absoluteresistance as Za, Zb and Zc. With this arrangement the L and R audiooutput signals at the receiver will be restored to the same amplituderelationship they had prior to L-R enhancement at the transmitter.

Although the 9.6 db (factor of 3) enhancement of the L-R channel hasbeen shown and discussed, it will be appreciated that by selecting othervalues for Zb, Zb, and Zc at the studio, a greater or lesser enhancementof the L-R component can be achieved. For example, for the factor of 2(6 db) enhancement illustrated by curve G of FIG. 16, it is necessarythat R_(L) =2R_(b) =2R_(a) with the result that ##EQU2##

As described earlier, curve G results from the combination of a 6 dbenhancement, with a 25 microsecond audio pre-emphasis then applied priorto multiplexing, followed by an additional pre-emphasis applied to thecomposite stereo signal after multiplexing. As shown in FIG. 18A theenhancement and audio pre-emphasis may be accomplished by means of acircuit 170 situated ahead of the system stereo multiplex generator 25(assuming no pre-emphasis within the generator), while a pre-emphasisnetwork 171 for the composite signal may be located between themultiplex generator and the transmitter modulator/amplifier circuits 30.For curve G the composite signal pre-emphasis time constant is 7.5microseconds.

Referring to FIG. 18B, to compensate at the receiver for thepre-emphasis of the composite signal introduced at the transmitter, a7.5 microsecond de-emphasis network 172 may be incorporated ahead of thedemodulator 55. Audio de-emphasis and the earlier described audiode-enhancement circuits of the receiver may be incorporated in a circuit173 at the output of demodulator 55.

It will be appreciated that the composite signal conditions representedby curves F and G of FIG. 16 have been presented as a means ofillustrating various techniques which may be combined in various degreesto shape the composite signal spectrum so as to optimize stereophonicperformance. It is anticipated that such techniques would ultimately bedefined as parameters in a yet to be adopted stereophonic televisionsound standard.

It should be understood that the curves of FIG. 16 are idealizations, inthat they represent the case for sinusoidal signals of prescribedamplitude plotted one frequency at a time. By contrast, audio programsoriginate as complex signals of constantly changing amplitude andfrequency. Modulation envelopes of complex signals tend to blend as acontinuum and to thereby obscure details of the underlying signals suchas the 8 KHz gap which separates the L+R and L-R components in FIG. 16.For this reason, observed complex signal modulated spectra for thestereophonic television sound transmission system appear somewhat asshown in the expanded television sound channel portrayed in FIG. 5.Referring to that figure, modulation of the sound carrier with acomposite signal, such as developed in the basic (non-enhanced)transmission system of this invention, results in the generation of anRF signal 180 at the upper end of the television channel (FIG. 2) havinga maximum overall bandwidth of 50 KHz during monophonic (L+R only)transmission, and an RF signal 181 having a maximum overall bandwidth of106 KHz during stereophonic transmissions. Modulation of the soundcarrier 26 with the composite signal developed when the basictransmission system has been modified to incorporate the circuitry ofFIG. 17A, so as to provide 9.6 db enhancement of the L-R component,results in generation of an RF signal 182 having shoulders above thoseof the envelope of RF signal 181.

The conditions depicted in FIGS. 3 and 16 represent extreme modulationlimits for the L+R and L-R components, those limits being mutuallyexclusive in the sense of being attainable in only one channel at atime, and only under the uncommon circumstance that the other channelequates to 0 at that instant. For this reason, most of the audio contentof the program is developed at levels below these limits, beingsusceptible therefore to environmental electrical noise to a stillgreater degree. One way to further improve noise rejection for the lowlevel signals is to process the program content before transmission in amanner that raises the average modulation of the composite signal closerto 100%, and to then compensate at the receiver with reciprocallymatched circuitry to restore the original conditions. One such system,the Dolby Type B noise reduction system, is finding increasingcommercial application in FM stereophonic radio broadcasting forreducing high frequency over-modulation and raising the averagemodulation level of the transmitted program.

Basically, the transfer characteristic of the Dolby B system is such asto enhance low-level high frequency signals, the degree of low levelenhancement increasing as a function of frequency. Since the Dolby Bsystem has been detailed prominently in the literature, only therelevant, qualitative features are noted herein. These are summarized inTable 2, wherein various configurations, consisting of varioustransmission pre-emphasis and reception de-emphasis time constants arecompared with and without Dolby Type B transmission and reception unitswith respect to maximum modulation level for high fidelity transmission(0 db = 100% modulation), high fidelity capability, and relativesignal-to-noise performance.

                                      TABLE 2                                     __________________________________________________________________________                         Maximum                                                  Configuration        Relative    Net                                          System                                                                             Transmitter                                                                           Receiver                                                                              Modulation                                                                          High  Relative                                     No.  Pre-Emphasis*                                                                         De-Emphasis*                                                                          Level Fidelity                                                                            S/N                                          __________________________________________________________________________    1    75      75      -8.3                                                                              db                                                                              Yes   -0.9 db                                      2    25      75      0   db                                                                              No    <2.7 db                                      3    25      25      0   db                                                                              Yes   +2.7 db                                      4    Dolby & 75                                                                            75      -8.3                                                                              db                                                                              No     --                                          5    Dolby & 25                                                                            75      0   db                                                                              No     --                                          6    Dolby & 25                                                                            25      0   db                                                                              No     --                                          7    Dolby & 25                                                                            25 & Dolby                                                                            0   db                                                                              Yes   +12.3 db                                     __________________________________________________________________________      *= microseconds                                                         

As can be seen in Table 2, the configurations are listed in order ofincreasing signal-to-noise (S/N) ratio, although, for configurations 4,5 and 6 the improvement has dubious merit, since the result isdistortion of the audio signal due to over-emphasis of high frequencies.Only configurations 1, 3, and 7, for which the de-emphasis is trulycomplementary, are capable of high fidelity reproduction; and of these,only configurations 3 and 7 can transmit all of the program material atessentially full modulation. Configuration 3 was described earlier asthe preferred 25 microsecond pre-emphasis/de-emphasis system for thebasic transmission/reception system of the invention. Configuration 7,which incorporates complementary Dolby noise-reduction, may beimplemented in the system of configuration 3 without modification,resulting in a 13.2 db S/N improvement as compared with thecomplementary 75 microsecond pre-emphasis standard for FM broadcasts.Furthermore, without system modification, Dolby Type B noise reductionmay be combined with the previously described L-R enhancement andcomposite signal pre-emphasis techniques for further noise reduction tothe extent permitted by whatever frequency deviation limits areultimately adopted as a standard for stereophonic television soundtransmissions.

FIG. 19A shows the application of Dolby Type B noise reduction to thebasic transmission system of the invention, wherein two Dolby Bprocessors 185 and 186 at the transmitter encode the pre-emphasized Land R audio signals prior to application to the system stereo generator25. At the receiver, as shown in FIG. 19B, the L and R audio outputs ofthe receiver stereo demodulator stage 55 are applied through respectivede-emphasis networks 187 and 188, which in accordance with the previousdiscussion have a time constant of 25 microseconds, to respective DolbyB processors 189 and 190, which decode the L and R audio signals. Thede-emphasis networks and processors together provide compensation torestore the de-emphasized L and R audio signals from demodulator 55 totheir original relationship.

The encoding and decoding processors may be basically identical inconstruction, differing only in the manner in which the input signal isrouted, as shown in FIG. 19C. The input signal traverses two paths; amain path through a combining network 191 and an inverter 192, and asecondary path through a voltage-responsive variable-frequency filter193, a signal amplifier 194, and an overshoot suppressor 195. The mainpath passes the input signal essentially unchanged. The secondary pathis essentially a filter which passes only low-level, high-frequencycomponents of the input signal. During encoding, the output of thesecondary path is added to the main path, boosting the low-level,high-frequency portions of the input signal. During decoding, the outputof the secondary path is subtracted from the main signal path output, aresult of the secondary path input being sensed as the inverted outputof the processor. Decoding thus removes the same information to the samedegree as was inserted during encoding.

The characteristics of the secondary path variable filter 193 aredetermined by a feedback loop consisting of a control amplifier 196 andrectifier/integrator 197. For signal amplitudes which do not exceed afixed threshold, no feedback signal is generated, and the transferfunction of the filter is simply that of a fixed, 500 Hz high passfilter. The threshold level is fixed at approximately 40 db below Dolbylevel, an internationally standardized reference corresponding to afrequency deviation of ± 37.5 KHz for FM broadcasting. A similarreference would be established for television sound at 50% of themaximum frequency deviation allowed for the television sound carrier.The gain of control amplifier 196 is a non-linear function of frequency,so that as signal amplitudes increase above the threshold level negativefeedback raises the variable filter cut-off frequency in a non-linearmanner, reaching a constant, maximally restricted bandwidth for inputsignals near Dolby level. The overall effect is negligible at lowfrequencies and at levels approaching full modulation, but increaseswith increasing frequency and decreasing amplitude.

In another embodiment of the invention, enhancement of low level signalsis accomplished in a manner similar to that of the Dolby Type B system,in that the degree of enhancement also increases as a function offrequency. However, unlike Dolby Type B, enhancement of low levelsignals occurs only above 20 KHz, and is applied to the compositesignal, being provided immediately following the system stereo multiplexgenerator 25, as with the pre-emphasis network 171 provided for thecomposite signal in FIG. 18A. Complementary circuitry is installed atthe receiver just prior to the stereo demodulator 55, as with thecomposite signal de-emphasis circuits 172 in FIG. 18B. Applied in thisway, only the L-R channel is affected, the effect being to raise themodulation level (and hence the noise tolerance) of low level L-Rsignals without increasing the modulation level of high-level signals.

Since neither commercial stereophonic television broadcasts nor suitablecommercial receivers for receiving such broadcasts are in existence, noproblem of obsolescence of existing equipment exists in adopting theproposed compatible transmission system. Receivers or adapters toreproduce television stereophonic sound in accordance with thetransmission/reception system of the invention may be manufactured withthe preferred 25 microsecond time constant, and may in fact immediatelyincorporate Dolby Type B reception circuitry, since such circuitry isalready commercially available in economical IC packaging. As forcompatibility with existing monophonic television receivers which have75 microsecond sound de-emphasis, it is doubtful that any unfavorablechange in the reproduced sound signal could be perceived, since very fewtelevision receivers are capable of high fidelity reception. At anyrate, FM broadcasts using the previously discussed configuration 5system indicate that many monophonic TV listeners would actuallyperceive the sound quality as improved because of its increased highfrequency content.

To better enable Dolby-B processor-equipped receivers to receivenon-Dolby transmissions a remote switching signal may be added to thecomposite signal to control the decoding processors within thereceivers. This switching signal may take the form of a sub-sonic fixedfrequency signal in the 10 to 25 Hz range which may be selectively addedto either the composite signal output of the stereo generator 25(FIG. 1) or generated within the stereo generator whenever thetransmission includes Dolby-B processing. The subsonic tone would bedetected by a control tone sensing circuit 198 (FIG. 19c) in thereceiver and utilized to control the Dolby-B processing stage(s)therein.

For example, a subsonic 20 Hz signal could be generated at a fixedamplitude corresponding to 25 Hz frequency deviation of the soundcarrier. The corresponding modulation index (1.25) of this inaudiblecomponent would be 60 db below the 100% modulation level (25 KHz), beingtherefore innocuous to other intelligence and functions. At thereceiver, a narrow-band frequency detector would respond to this 20 Hzcomponent by generating a DC control signal suitable for switching orotherwise conditioning the receiver for Dolby-B decoding.

The above-described subsonic switching signal requires no additionalbandwidth and, combined with the control possibilities of the 19 Khzpilot component constitutes a flexible means for conveying conditions oftransmission. This is shown in Table 3 which illustrates the fourtransmission format conditions possible with the pilot and subsonicsignals.

                  TABLE 3                                                         ______________________________________                                        Control Signals                                                               Pilot Subsonic Signal  Transmission Format                                    ______________________________________                                        Yes   No             Stereophonic, 25 Microsecond                                                  preemphasis                                              No    No             Monophonic, 75 Microsecond                                                    preemphasis                                              No    Yes            Monophonic, Dolby, 25 Micro-                                                  second preemphasis                                       Yes   Yes            Stereophonic, Dolby with 25                                                   Microsecond preemphasis                                  ______________________________________                                    

It will be appreciated that more than one subsonic switching signal maybe transmitted to accomplish control functions, and that theabove-described subsonic switching technique is also applicable toconventional FM stereo broadcast transmissions.

The system of the present invention enables sterophonic sound to bebroadcast over commerical television channels and faithfully andcompatably reproduced in conjunction with conventional existingtelevision receivers. The system requires a minimal amount of additionaltransmitting equipment and minimal modification of existing receivingequipment. With modification the system may also provide for compatablebroadcast and reception of bilingual television programming.

Television sound transmissions in accordance with the invention may bereceived on converters, either of the type wherein channel selection isaccomplished independently of the associated television receiver onwhich video information is being reproduced, or by means of the tunercontained in the receiver. Such converter may provide a low level audiosignal for amplification on an external stereo amplifier/speaker system,or may provide high level audio and/or speakers for direct soundreproduction. A variation of the converter allows reception by means ofa conventional stereo FM broadcast receiver. Alternatively, adapter maybe integrally installed in existing or newly constructed televisionreceivers to achieve stereophonic sound reception.

Improvements in the signal-to-noise ratio of the basic transmissionsystem are possible by

1. Adopting a 25 microsecond preemphasis/deemphasis time constant,

2. Enhancing the L-R component of the composite signal,

3. Applying preemphasis and deemphasis to the composite signal,

4. Applying Dolby Type-B signal processing to the L and R audio signals,and/or

5. Applying Dolby Type-B signal processing to the L-R component of thecomposite signal. Further improvement is contemplated by selecting apilot component having a frequency equal to 5/4 F_(H) to reduce thenumber of audible interference bands which result from horizontalscanning frequency harmonics within the L-R component sidebands.

I claim:
 1. A stereophonic sound transmission system for transmittingwithin a television broadcast channel having defined frequency limitsleft and right audio source signals and a video source signal eachhaving a predetermined upper frequency limit, said video source signalbeing amplitude-modulated on a video carrier to form an AM signaloccupying a first portion of said channel, said system comprising, incombination:generator means including a stereo multiplex generator forgenerating a composite signal including a first component representativeof the sum of said left and right source signals, a secondamplitude-modulated subcarrier component representative of thedifference between said left and right signals, said subcarriercomponent having upper and lower sidebands centered about a suppressedsubcarrier, and a pilot component having a frequency one-half thefrequency of said suppressed subcarrier; transmitter means includingmeans for generating a sound carrier within said broadcast channel, andmeans for frequency-modulating said sound carrier with said compositesignal to form an FM signal occupying a second portion of said channel;receiver means for receiving said television broadcast channel includingmeans for deriving said composite signal; and demodulator means fordeveloping said left and right audio source signals from said compositesignal.
 2. A stereophonic sound transmission system as defined in claim1 wherein said sound carrier is spaced by 4.5 MHz from said videocarrier.
 3. A stereophonic sound transmission system as defined in claim1 wherein said second component of said composite signal is spaced fromsaid sound carrier by at least twice the maximum upper frequency limitof said audio source signals.
 4. A stereophonic sound transmissionsystem as defined in claim 3 wherein said upper limit of said left andright audio source signals is approximately 15 KHz, the frequency ofsaid suppressed subcarrier is 38 KHz, the frequency of said pilotcomponent is 19 KHz.
 5. A stereophonic sound transmission system asdefined in claim 1 wherein said video source signal has a predeterminedhorizontal scanning frequency, the frequency of said pilot component is5/4 said horizontal scanning frequency, and the frequency of saidsubcarrier is twice the frequency of said pilot component.
 6. Astereophonic sound transmission system as defined in claim 1 whereinsaid upper and lower sidebands each cause a maximum frequency deviationto said frequency-modulated carrier equal to one-half the maximumdeviation caused by said first component.
 7. A stereophonic soundtransmission system as defined in claim 6 wherein said first componentcauses a maximum frequency deviation of 22.5 KHz to saidfrequency-modulated sound carrier and said upper and lower sidebands ofsaid second component each cause a maximum frequency deviation of 11.25KHz to said sound carrier.
 8. A stereophonic sound transmission systemas defined in claim 7 wherein said pilot component causes a maximumdeviation of 2.5 KHz to said frequency-modulated carrier.
 9. Astereophonic sound transmission system as defined in claim 1 whereinsaid multiplex generator includes a pair of pre-emphasis networks forimposing pre-emphasis on respective ones of said left and right audiosource signals, and wherein said demodulator means include a second pairof complementary de-emphasis networks for imposing compensatingde-emphasis on said developed left and right audio source signals.
 10. Astereophonic sound transmission system as defined in claim 9 whereinsaid pre-emphasis and de-emphasis networks have a time constant ofapproximately 25 microseconds.
 11. A stereophonic sound transmissionsystem as defined in claim 1 which further includes a pre-emphasisnetwork disposed between said generator means and said transmittermeans, and a complementary de-emphasis network disposed between saidreceiver means and said demodulator means.
 12. A stereophonic soundtransmission system as defined in claim 11 wherein said networks have atime constant of approximately 7.5 microseconds.
 13. A stereophonicsound transmission system as defined in claim 12 wherein said generatormeans include circuit means for enhancing said second component by afactor of two with respect to said first component, and said demodulatormeans include complementary de-enhancement circuit means for enhancingsaid first component by a factor of two with respect to said secondcomponent.
 14. A stereophonic sound transmission system as defined inclaim 13 which further includes a pair of additional pre-emphasisnetworks between said left and right sources and said multiplexgenerator, and wherein said demodulator means include a pair ofadditional complementary de-emphasis networks for the left and rightaudio source signals developed therein.
 15. A stereophonic soundtransmission system as defined in claim 14 wherein said additionalnetworks have a time constant of 25 microseconds.
 16. A stereophonicsound transmission system as defined in claim 1 wherein said generatormeans include circuit means for enhancing said second component withrespect to said first component, and said demodulator means includecomplementary deenhancement circuit means.
 17. A stereophonic soundtransmission system as defined in claim 16 wherein said second componentis enhanced by a factor of 3 and deenhanced by a factor of
 3. 18. Astereophonic sound transmission system as defined in claim 16 whereinsaid enhancement circuit means include first and second differentialamplifiers, each having inverting and non-inverting input terminals,said left and right audio source signals being applied to saidnon-inverting terminals of said first and second amplifiers,respectively, and matrixing means comprising a first impedance connectedbetween said inverting terminals, and second and third impedancesconnected between the outputs of said amplifiers to respective ones ofsaid inverting terminals, said enhanced left and right audio sourcesignals being developed at the outputs of respective ones of saidamplifiers.
 19. A stereophonic sound transmission system as defined inclaim 16 wherein said deenhancement circuit means include first andsecond amplifiers, and matrixing means comprising first and secondimpedances serially connected between the left and right audio signalsdeveloped by said demodulator and the input terminals of respective onesof said amplifiers, and a third impedance connected between said inputterminals.
 20. A stereophonic sound transmission system as defined inclaim 1 which further includes a pair of Dolby-B encoders coupledbetween said left and right audio sources and respective inputs of saidmultiplex generator, and a pair of Dolby-B decoders coupled torespective left and right audio source outputs of said demodulatormeans.
 21. A stereophonic sound transmission system as defined in claim20 which further includes a pair of pre-emphasis networks seriallydisposed with said Dolby-B encoders between said audio sources and saidstereo generator, and a pair of de-emphasis networks serially disposedwith said Dolby-B decoders at the output of said decoding means.
 22. Astereophonic sound transmission system as defined in claim 21 whereinsaid pre-emphasis and de-emphasis networks have a 25 microsecond timeconstant.
 23. A stereophonic sound transmission system as defined inclaim 20 which further includes means for switching said Dolby-Bencoders in and out of circuit, mode control means for adding a controlsignal component to said composite signal, and means responsive to saidcontrol effect associated with said receiver means for selectivelyswitching said Dolby-B decoders in and out of circuit.
 24. Astereophonic sound transmission system as defined in claim 23 whereinsaid left and right audio source signals each have a lower frequencylimit, and said control signal component comprises a continuous-wavesignal below said lower frequency limit.
 25. A stereophonic soundtransmission system as defined in claim 1 wherein said generator meansinclude encoding means providing a transfer characteristic whichincreases the amplitude of low-level high frequency signals, the degreeof increase decreasing with amplitude and increasing as a function offrequency, and said demodulator means include decoding circuit meanshaving a complementary characteristic.
 26. A stereophonic soundtransmission system defined in claim 25 wherein said transfercharacteristic substantially affects only signals above 20 KHz andextends substantially only to 53 KHz.
 27. A stereophonic soundtransmission system as defined in claim 2 wherein said left and rightaudio source signals have an upper limit of 15 KHz, said suppressedcarrier is centered at 38 KHz, and said frequency-modulated signal has amaximum overall bandwidth of 106 KHz.
 28. The method of transmittingwithin a television broadcast channel having defined frequency limitsleft and right audio source signals and a video source signal eachhaving a predetermined upper frequency limit, said video source signalbeing amplitude modulated on a video carrier to form an AM signaloccupying a first portion of said channel, comprising the stepsof:generating a composite signal including a first componentrepresentative of the sum of said left and right source signals, asecond amplitude-modulated subcarrier component representative of thedifference between said left and right signals, said subcarriercomponent having upper and lower sidebands centered about a suppressedsubcarrier, and a pilot component having one-half the frequency of saidsuppressed subcarrier; generating an RF sound carrier in said broadcastchannel having a predetermined frequency spacing from saidvideo-modulated signal; modulating said sound carrier with saidcomposite signal to form an FM signal occupying a second portion of saidchannel; conveying said video- and audio-modulated signals in saidbroadcast channel to a receiving location, and at said receivinglocation, developing said composite signal from said audiomodulatedsignal in said channel; and developing said left and right audio sourcesignals from said composite signal.
 29. The method of transmittingstereophonic sound as defined in claim 28 wherein said sound carrier isseparated from said video carrier in said channel by 4.5 MHz.
 30. Themethod of transmitting stereophonic sound as defined in claim 28 whereinsaid video source signal has a predetermined horizontal scanningfrequency, the frequency of said pilot component is 5/4 said horizontalscanning frequency, and the frequency of said subcarrier is twice thefrequency of said pilot component.
 31. The method of transmittingstereophonic sound as defined in claim 28 wherein said upper and lowersidebands each cause a maximum frequency deviation to saidfrequency-modulated carrier equal to one-half and maximum deviationcaused by said first component.
 32. The method of transmittingstereophonic sound as defined in claim 31 wherein said first componentcauses a maximum frequency deviation of 22.5 KHz to saidfrequency-modulated sound carrier and said upper and lower sidebands ofsaid second component each cause a maximum frequency deviation of 11.25KHz to said sound carrier.
 33. The method of transmitting stereophonicsound as defined in claim 28 comprising the additional stepsof:introducing pre-emphasis to said left and right audio source signalsat the transmitter location, and introducing complementary de-emphasisto said developed left and right audio signals at said receiverlocation.
 34. The method of transmitting stereophonic sound as definedin claim 33 wherein said pre-emphasis and de-emphasis have 25microsecond time constants.
 35. The method of transmitting stereophonicsound as defined in claim 28 which includes the further stepsof:applying preemphasis to said composite signal at the transmitterlocation; and applying complementary deemphasis to said composite signalat the receiver location.
 36. The method of transmitting stereophonicsound as defined in claim 35 wherein said preemphasis has a timeconstant of 7.5 microseconds.
 37. The method of transmittingstereophonic sound as defined in claim 28 which includes the furthersteps of:enhancing the level of said component with respect to saidfirst component at the transmitter location; and complementarilydeenhancing the level of said second component with respect to saidfirst component at the receiver location.
 38. The method of transmittingstereophonic sound as defined in claim 37 wherein said second componentis enhanced by a factor of
 2. 39. The method of transmittingstereophonic sound as defined in claim 28 which includes the furthersteps of:applying Dolby-B type encoding to said left and right audiosource signals at said transmitter; and applying complementary Dolby-Btype decoding to said left and right audio output signals at saidreceiver.
 40. The method of transmitting stereophonic sound as definedin claim 28 which includes the further steps of:increasing low-levelhigh-frequency portions of said composite signal at the transmitterlocation, said increase having a magnitude increasing with frequency anddecreasing with amplitude; and having an upper frequency limitsubstantially corresponding to said upper frequency limit of said L-Rsignal; and complementarily decreasing said composite signal at thereceiver location.
 41. The method of transmitting stereophonic sound asdefined in claim 40 wherein only portions of said composite signalsubstantially between 20 KHz and 53 KHz are increased.
 42. A receiverfor receiving stereophonic sound transmissions included on a televisionbroadcast channel of defined frequency limits, wherein said soundtransmissions comprise a sound carrier frequency-modulated by acomposite signal including a first component representative of the sumof the left and right source signals, a second amplitude modulatedsubcarrier component representative of the difference between the leftand right signals, said subcarrier component having upper and lowersidebands centered about a suppressed subcarrier, and a pilot componenthaving a frequency one-half the frequency of said suppressed carrier;said receiver comprising, in combination:tuner means for converting saidtelevision broadcast channel to an intermediate frequency channelincluding an intermediate frequency sound signal; sound bandpass filtermeans for separating said sound signal from said intermediate frequencychannel; sound detector means for deriving from said intermediatefrequency sound signal a composite signal including said first, secondand third components; and stereo demodulator means for deriving saidleft and right source signals from said composite signal.
 43. A receiveras defined in claim 42 wherein said intermediate frequency sound signalis centered at 10.7 MHz.
 44. A receiver as defined in claim 42 whereinsaid tuner means comprise the tuner of an associated televisionreceiver.
 45. A receiver as defined in claim 42 which includes secondconversion means comprising a mixer stage and a continuous waveoscillator, and wherein said intermediate frequency sound signal isconverted by said conversion means to a second intermediate frequencyand said sound intermediate frequency signal is applied to saiddetector.
 46. A receiver as defined in claim 45 wherein said firstintermediate frequency is 41.25 MHz and said second intermediatefrequency is 10.7 MHz.
 47. A receiver as defined in claim 42 whereinsaid intermediate frequency channel includes an intermediatefrequencyvideo signal; and which further includes:video bandpass filter means forseparating said video signal from said intermediate frequency channel;Fm detector means for deriving from said intermediate frequency videosignal an output signal representative of frequency shift in said videosignal; and matrixing means for combining said output signal with saidfirst component of said composite signal to compensate for frequencyshift in said intermediate frequency channel.
 48. A receiver as definedin claim 47 which includes first and second mixer stages and wherein theoutput of said sound bandpass filter is applied to said first mixerstage to develop a sound signal at a second intermediate frequency, saidsound signal being applied to said sound detector, and wherein theoutput of said video bandpass filter is applied to said second mixerstage to develop an intermediate frequency video signal, and saidintermediate frequency video signal is applied to said FM detectormeans.
 49. A receiver as defined in claim 48 which includes a continuouswave oscillator, the output of said oscillator being applied to saidfirst and second mixer stages to develop said second intermediatefrequency sound and video signals.
 50. A receiver as defined in claim 49wherein said first intermediate frequency video signal is centered at45.75 MHz, said first intermediate frequency audio signal is centered at41.25 MHz, said second intermediate frequency video signal is centeredat 6.2 MHz, said second intermediate frequency sound signal is centeredat 10.7 MHz, and the frequency of said oscillator is 51.95 MHz.
 51. Atelevision sound converter for receiving on an FM stereo broadcastreceiver adapted to receive signals having a predetermined maximumdeviation within a predetermined frequency range stereophonic soundtransmissions included on a television broadcast channel of definedfrequency limits, wherein said sound transmissions comprise a soundcarrier frequency-modulated by a composite signal including a firstcomponent representative of the sum of the left and right sourcesignals, a second amplitude-modulated subcarrier componentrepresentative of the difference between the left and right signals,said subcarrier component having upper and lower sidebands centeredabout a suppressed carrier, and a pilot component representative of thephase and frequency of the suppressed carrier; said convertercomprising, in combination:tuner means for converting said televisionbroadcast channel to an intermediate frequency channel including anintermediate frequency sound signal, said signal having a frequencycomprising a predetermined fraction of a predetermined receptionfrequency within said predetermined frequency range, and having amaximum frequency deviation equal to said predetermined fraction of saidpredetermined maximum deviation of said receiver, where said fractionhas a numerator of unity and a denominator of integer n; frequencymultiplier means for multiplying the frequency of an applied signal by afactor of n; and means for applying said sound signal to said frequencymultiplier means to obtain an RF output signal for application to saidFM broadcast receiver.
 52. A television sound converter as defined inclaim 51 wherein said predetermined fraction is 1/3, said predeterminedmaximum deviation of said receiver is 75 KHz, and said predeterminedmaximum deviation of said intermediate frequency signal is 25 KHz.
 53. Atelevision sound converter as defined in claim 52 wherein saidintermediate frequency signal is centered at 30 MHz and tripled to 90MHz by said frequency multiplier means for application to said FM stereobroadcast receiver.
 54. A system for simultaneously transmitting firstand second audio program sources in conjunction with a video sourcesignal modulated on an RF carrier on a television broadcast channelhaving defined frequency limits, said system comprising, incombination:generator means including a stereo multiplex generator forgenerating from left and right applied input signals a composite signalincluding a first component representative of the sum of said left andright signals, a second amplitude-modulated subcarrier componentrepresentative of the difference between said left and right signals,said subcarrier component having upper and lower sidebands centeredabout a suppressed subcarrier, and a pilot component representative ofthe phase and having one-half the frequency of said suppressedsubcarrier; means for generating an RF sound carrier within saidbroadcast channel having a predetermined spacing from saidvideo-modulated carrier, means for frequency-modulating said soundcarrier with said composite signal; receiver means for receiving saidtelevision broadcast channel including means for deriving said compositesignal; demodulator means for developing said first and second audiosignals from said composite signal; first matrixing means associatedwith said generator means for combining said first and second audioprogram sources to form said left and right input signals whereby saidfirst component of said composite signal conveys only said first programsource and said second component of said composite signal conveys onlysaid second program source; and second matrixing means associated withsaid demodulator means for combining said left and right audio signalsto form said first and second program sources, respectively.
 55. Atransmission system as defined in claim 54 wherein said first matrixingmeans comprise a pair of differential amplifiers; said first audiosource being coupled through first and second impedances to theinverting imputs of said first and second amplifiers, respectively, andsaid second audio source being coupled through a third impedance to theinverting imput of said first amplifier and through a fourth impedanceto the non-inverting imput of said second amplifier, the output of saidfirst and second amplifiers comprising said left and right audiosources, respectively.
 56. A transmission system as defined in claim 54wherein said second matrixing means comprises a differential amplifierand left and right audio source means including a pair of impedances forcoupling said source signals developed by said demodulator to thenon-inverting and inverting imputs of said amplifier, the output of saidamplifier comprising said second audio program source.
 57. Atransmission system as defined in claim 56 wherein said demodulator canbe selectively operated in a monaural mode wherein the output of saiddemodulator comprises said first audio program source, and whereinuser-actuable means are provided for conditioning said demodulator tooperate in said monaural mode to obtain reception of said first audioprogram source.
 58. A transmission system as defined in claim 54 whereinsaid first and second audio program sources comprise first and seconddifferent languages, respectively.
 59. The method of transmittingstereophonic sound as defined in claim 28 wherein said left and rightaudio source signals have upper frequency limits of 15 KHz, saidsubcarrier component is centered at 38 KHz, and said pilot component iscentered at 19 KHz.